Question

Running this batch file

@echo off
set a=some value with (parentheses) inside
if 1 == 1 (
    set PATH=%a%
)

gives inside was unexpected at this time. error.

How to escape a variable to avoid this error?

Answer 1

You can use two different ways

Use the extended syntax of set with quotes set "var=content" will set var with content,
content is quoted so special characters aren’t problematic and it uses the content till the last quote (without the quote itself)

@echo off
set a=some value with (parentheses) inside
if 1 == 1 (
    set "PATH=%a%"
)

Use delayed expansion (like the answer of shf301) but also transfer the value to the main scope.

@echo off
setlocal enabledelayedexpansion
set a=some value with (parentheses) inside
if 1 == 1 (
    set "localScope_PATH=!a!"
    rem now transfer it to the global scope
    FOR /F "delims=" %%A in ("!localScope_PATH!") DO (
       endlocal
       set "path=%%A"
    )
)

In this case the extended set-syntax is not necessary, I used it only to avoid hidden spaces at the line end.

EDIT:
Can I combine this with setlocal EnableDelayedExpansion and using ! instead of % to lazy evaluate variable’s value? When I tried I got )! was unexpected at this time.

You can, but it’s contra productive, as

@echo off
Setlocal EnableDelayedExpansion
set a=some value with (parentheses) inside
if 1 == 1 (
    set PATH=!a:^)=^^^)!
    set path
)

Then your path contains carets in front of the ) like
C:\programs (x86^)

To understand how expansion works you can read SO:How does the Windows Command Interpreter (CMD.EXE) parse scripts?

EDIT2: More problems with the path (containing quotes)
According to this question there can occour another problem with parenthesis when the path contains quotes.

Sample
path="C:\Program Files (x86)";C:\Program Files (x86)\Skype

This is allowed, even it’s not necessary to use quotes here, but this destroys the extended SET syntax, as now set "newPath=%path%" expands to

set "newPath="C:\Program Files (x86)";C:\Program Files (x86)\Skype"

Now at least one parenthesis is not inside quotes and is able to break a command block.

But you can simply remove all quotes from the path variable, as said, quotes aren’t necessary here.

set "newPath=%path:"=%"